Highly porous biochar from spent coffee ground for fully green thermal insulating composites with thermal conductivity of 0.04 W m⁻¹ K⁻¹

Biomass materials offer the advantages of low cost, abundant availability, and renewability, making them sustainable and environmentally friendly alternatives to petroleum-based thermal insulation materials. Spent coffee ground (SCG), a type of biomass generated after brewing coffee, has been used as a filler for thermal insulation composites. However, developing SCG-based composites with superior thermal insulation performance remains a significant challenge due to the low porosity of SCG. Herein, a highly porous SCG biochar (SB) was fabricated by a simple carbonization process optimized for temperature and atmosphere. SB-based thermal insulation polymer composites were designed and fabricated for the first time, employing the SB as the filler and biodegradable ethyl cellulose (EC) as the matrix. Meanwhile, the SB was pre-mixed with environmentally friendly solvents (water, ethanol, and propylene glycol) selected based on the chemical interaction within the material system to prevent the impregnation of SB pores with the EC. The chemical and morphological properties of the SB were discussed in terms of their influence on the thermal insulation properties of the fabricated composite. The thermal conductivity and heat transfer mechanism of the SB-based composites were systematically verified using steady-state heat transfer theory. The biodegradable SB-based composite demonstrated outstanding thermal insulation property (0.04 W m⁻¹ K⁻¹) comparable to commercial expanded polystyrene. More importantly, the thermal insulation composites developed in this work consisted of environmentally friendly and sustainable materials, free from hazardous or toxic substances. These thermally insulating and biodegradable SB-based polymer composites are expected to be widely utilized in various thermal insulation applications.